This invention relates to the extrusion of polymeric resins and like materials, and more particularly, to a co-extrusion head which efficiently presents a concentric, multi-layer, tubular product to a downstream die head assembly.
Various cross head and die assemblies have heretofore been utilized in an attempt to present a concentric, tubular extrusion of first and second thermoplastic resins to a downstream die assembly. These devices receive thermoplastic resins from upstream sources which are heated and under pressure. Such resins enter a cross head and die assembly and are usually fed into resin distribution channels for downstream flow.
One problem with known prior art devices, presenting multi-layer tubular products, is that the resin or melt for the outside layer enters the assembly from an offset position relative to an annular channel. The resin flowing in the channel is not geometrically balanced as portions of the resin flow must necessarily travel different distances to points around the channel due to the offset inlet port. This geometric imbalance results in an unstable resin flow. In order to stabilize the flow the velocity of the flow must be adjusted. However, the velocity of a resin flow is difficult to adjust absent an increase in temperature and pressure. Such increases can also result in an unstable resin flow. Moreover, at elevated temperatures and/or pressure, the resin may burn causing aberrations in the finished product.
A geometric imbalance in the resin or melt, if not corrected or compensated, can present an uneven wall thickness in the final tubular product. An imbalanced melt flow can cause turbulence which can create conditions disrupting a preferred laminar flow of the resin. Such disruptive flow conditions can result in further aberrations such as an undesirable weld/rupture line in the finished product, rough surfaces on the finished product and discolorations along the surface of the final product. Thus, absent a continuous and special operator attention to the resin flow, it is very difficult to obtain a desired uniform quality in the finished product.
In response thereto I have invented a co-extrusion head for a die assembly which provides for a geometrically balanced flow of first and second resins therethrough. My device generally comprises a melt housing having upstream and downstream faces with a first resin input port in the upstream face. An annular channel extends from the input port and increases in diameter upon its progression through the melt housing. The first resin melt is discharged through an annular outlet on the downstream face at a diameter greater than its input. The annular outlet and inlet ports share an imaginary common axis such that the intermediate annular channel is generally symmetrical about the axis. This structure presents a geometrically balanced channel relative to the inlet and outlet ports such that all the resin particles flowing through the channel travels through equidistant paths between the inlet and outlet apertures.
Concurrently the inner layer of resin material is injected into a second resin inlet on a lateral surface of the melt housing for flow through an intermediate bore crossing the first resin annular channel and discharge from an outlet port which is centrally located within the annular outlet for the first resin. Thus, a first resin flow surrounds a second resin flow upon discharge from the melt housing.
A spider member connects a bushing housing to the melt housing for continuing the above resin flows. The bushing housing includes a central bore having inlet and outlet apertures. Within the central bore is positioned a cone-shaped pin with central bore therein. The external surface of the pin cooperates with the surface of the bushing housing bore to present a channel for flow of the first resin mass therethrough. Concurrently the second resin mass flows through the pin bore for discharge therefrom. The shape of the pin shapes the configurations of the first and second resin mass flows so as to enhance a melding of the resin flows at the downstream pin tip. Accordingly, a tubular product having a first resin layer surrounding a tubular second resin mass is presented for downstream processing by a die inserted within the bushing and attached die head assembly.
It is therefor a general object of this invention to provide a co-extrusion head for efficiently presenting a multi-layer, tubular resin product to a downstream die assembly.
It is another general object of this invention to provide a co-extrusion head, as aforesaid, which presents geometrically balanced resin flows.
Still another object of this invention is to provide a co-extrusion head, as aforesaid, which efficiently melds first and second resin flows of a multi-layer resin product.
A further object of this invention is to provide a co-extrusion head, as aforesaid, which efficiently introduces first and second resin flows into a melt housing.
Another object of this invention is to provide a co-extrusion head assembly, as aforesaid, which provides first and second resin flows that can be easily monitored and efficiently conducted through the head at various temperatures and pressures.
A further object of this invention is to provide a co-extrusion head, as aforesaid, which can efficiently conduct resin flows therethrough at selected velocities.
Still another object of this invention is to provide a co-extrusion head, as aforesaid, which requires minimal tooling so as to connect the same with a downstream die insert and die assembly.
A particular object of this invention is to provide a co-extrusion head, as aforesaid, presenting a geometrically balanced flow of a first resin through a melt housing so as to surround a second resin flow upon discharge from said melt housing.
Another particular object of this invention is to provide a co-extrusion head, as aforesaid, presenting geometrically balanced first and second resin flows through a bushing housing so as to enhance a melding of the resin flows prior to discharge from the bushing housing.
A still further object of this invention is to provide a co-extrusion head, as aforesaid, utilizing a replaceable pin insert in said bushing housing so as to shape the first and second resin flows passing through the bushing housing.
Another particular object of this invention is to provide a pin element, as aforesaid, which can be changed according to the type of resin melt passing through the co-extrusion head.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, a now preferred embodiment of this invention.